Electric Field Lines and Dipoles
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Transcript of Electric Field Lines and Dipoles
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Assignment 3: Field Lines and Dipoles
Due: 8:00am on Wednesday, January 18, 2012
Note: To understand how points are awarded, read your instructor's Grading Policy.
[Switch to Standard Assignment View]
Visualizing Electric Fields
Learning Goal: To understand the nature of electric fields and how to draw field lines.
Electric field lines are a tool used to visualize electric fields. A field line is drawn beginning at a positive charge
and ending at a negative charge. Field lines may also appear from the edge of a picture or disappear at the edge of
the picture. Such lines are said to begin or end at infinity. The field lines are directed so that the electric field at
any point is tangent to the field line at that point.
The figure shows two different ways to visualize an electric
field. On the left, vectors are drawn at various points to show the direction and magnitude of the electric field. On
the right, electric field lines depict the same situation. Notice that, as stated above, the electric field lines are drawn
such that their tangents point in the same direction as the electric field vectors on the left. Because of the nature of
electric fields, field lines never cross. Also, the vectors shrink as you move away from the charge, and the electric
field lines spread out as you move away from the charge. The spacing between electric field lines indicates the
strength of the electric field, just as the length of vectors indicates the strength of the electric field. The greater the
spacing between field lines, the weaker the electric field. Although the advantage of field lines over field vectors
may not be apparent in the case of a single charge, electric field lines present a much less cluttered and more
intuitive picture of more complicated charge arrangements.
Part A
Which of the following figures correctly depicts the field lines
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from an infinite uniformly negatively charged sheet? Note that the sheet is being viewed edge-on in all pictures.
Hint A.1 Description of the field
Hint not displayed
ANSWER:
A
B
C
D
Correct
Part B
In the diagram from part A , what is wrong with figure B?
(Pick only those statements that apply to figure B.)
Check all that apply.
ANSWER:
Field lines cannot cross each other.
The field lines should be parallel because of the sheet's symmetry.
The field lines should spread apart as they leave the sheet to indicate the weakening of the
field with distance.
The field lines should always end on negative charges or at infinity.
Correct
Part C
Which of the following figures shows the correct electric field
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lines for an electric dipole?
ANSWER:
A
B
C
D
Correct
This applet shows two charges. You can alter the charge on each independently or alter the distance between
them. You should try to get a feeling for how altering the charges or the distance affects the field lines.
Part D
In the diagram from part C , what is wrong with figure D?
(Pick only those statements that apply to figure D.)
Check all that apply.
ANSWER:
Field lines cannot cross each other.
The field lines should turn sharply as you move from one charge to the other.
The field lines should be smooth curves.
The field lines should always end on negative charges or at infinity.
Correct
In even relatively simple setups as in the figure, electric field
lines are quite helpful for understanding the field qualitatively (understanding the general direction in which a
certain charge will move from a specific position, identifying locations where the field is roughly zero or where
the field points a specific direction, etc.). A good figure with electric field lines can help you to organize your
thoughts as well as check your calculations to see whether they make sense.
Part E
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In the figure , the electric field lines are shown for a system of
two point charges, and . Which of the following could represent the magnitudes and signs of and ?
In the following, take to be a positive quantity.
ANSWER:
,
,
,
,
,
Correct
Very far from the two charges, the system looks like a single charge with value . At large enough
distances, the field lines will be indistinguishable from the field lines due to a single point charge .
Torque on a Dipole in a Uniform Field
Consider an electric dipole whose dipole moment (a vector
pointing from the negitive charge to the positive charge) is oriented at angle with respect to the y axis. There is
an external electric field of magnitude (independent of the field produced by the dipole) pointing in the positive
y direction. The positive and negative ends of the dipole have charges and , respectively, and the two
charges are a distance apart. The dipole has a moment of inertia about its center of mass. It will help you to
imagine that the dipole is free to rotate about a pivot through its center.
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Part A
What is the net force that the dipole experiences due to the electric field?
Hint A.1 What is the force on the positive charge?
Hint not displayed
Express in terms of the given variables and the unit vectors , .
ANSWER:
=
Correct
Part B
What is , the magnitude of the torque that the electric field exerts about the center of mass of the dipole?
Hint B.1 Find the torque on the positive charge
Hint not displayed
Hint B.2 Now consider the negative charge
Hint not displayed
Express the magnitude of the total torque in terms of the given quantities.
ANSWER:
=
Correct
Part C
Using the above result, find the potential energy associated with the dipole's orientation in the field as a
function of the angle shown in the figure. Take the zero of the potential to occur when the dipole is at angle
; that is, .
Hint C.1 The definition of potential energy
Hint not displayed
Hint C.2 How to choose the constant of integration
Hint not displayed
Express in terms of the given quantities.
ANSWER:
=
Correct
The dipole moment of a neighboring pair of opposite charges of equal magnitude is defined to be a vector from
negative to positive charge of magnitude equal to the product of and the distance between the charges.
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Part D
In terms of the dipole moment , which of the following is an expression for the torque on a dipole in the field
?
Hint D.1 How to approach this part
Hint not displayed
Hint D.2 Formula for
Hint not displayed
Hint D.3 Formula for
Hint not displayed
Hint D.4 More on torques
Hint not displayed
ANSWER:
Correct
Part E
In terms of the dipole moment , which of the following is an expression for the potential energy of a dipole in
the field ?
Hint E.1 Formula for
Hint not displayed
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Hint E.2 Formula for
Hint not displayed
ANSWER:
Correct
Electric Dipole in an Electric Field
Point charges 4.00 and 4.00 are separated by distance 3.60 , forming an electric dipole.
Part A
Find the magnitude of the electric dipole moment.
Hint A.1 How to approach the problem
Hint not displayed
Hint A.2 Formula for dipole moment
Hint not displayed
Express your answer in coulomb meters to three significant figures.
ANSWER:
1.441011
Correct
Part B
What is the direction of the electric dipole moment?
ANSWER:
from to
from to
Correct
Part C
The charges are in a uniform electric field whose direction makes an angle 36.9 with the line connecting the
charges. What is the magnitude of this field if the torque exerted on the dipole has magnitude 7.40109
?
Hint C.1 Find an equation for the torque
Hint not displayed
Hint C.2 How to obtain
Hint not displayed
Express your answer in newtons per coulomb to three significant figures.
-
ANSWER:
856
Correct
Electrostatic Force of Water on a Chlorine Ion
The dipole moment of the water molecule is . Consider a water molecule located at the
origin whose dipole moment points in the positive x direction. A chlorine ion , of charge ,
is located at meters. Assume that this x value is much larger than the separation between the
charges in the dipole, so that the approximate expression for the electric field along the dipole axis can be used.
Part A
Find the magnitude of the electric force, ignoring the sign, that the water molecule exerts on the chlorine ion.
Hint A.1 How to approach the problem
Hint not displayed
ANSWER:
6.581013
Correct
Part B
What is the direction of the electric force?
ANSWER:
negative x
positive x
Correct
Since the dipole moment points in the positive x direction, the positive side of the dipole is to the right of the
negative side of the dipole. The electric field along the x axis due to the dipole will then point to the right
everywhere (except the small interval between the two charges of the dipole). Try to visualize the electric field of
a dipole if you need clarification, and think about which way the electric field lines point on each end (left and
right). Since the field will point to the right at the location of the chlorine ion, and since the ion is negatively
charged, the force exerted on the ion will be to the left, or along the negative x direction.
Part C
Is this force attractive or repulsive?
ANSWER:
attractive
repulsive
Correct
Since the ion is along the positive x axis, and the force is pointing in the negative x direction, the force is pulling
the chlorine ion toward the dipole (which is at the origin), and so the force is attractive.
Misprints in Exercise 21:60-- Refer to Example 21.14 and Figure 21.33
Exercise 21.60
Consider the electric dipole of Example 21.15.
Part A
-
Derive an expression for the magnitude of the electric field produced by the dipole at a point on the -axis in
Fig.21.34 in the textbook.
Express your answer in terms of the variables , , , and appropriate constants.
ANSWER:
= Correct
Part B
What is the direction of this electric field?
ANSWER:
= 270
Correct counterclockwise from the -direction
Part C
How does the electric field at points on the -axis depend on when is very large?
Express your answer in terms of the variables , , , and appropriate constants.
ANSWER:
= Correct
Dipole Motion in a Uniform Field
Consider an electric dipole located in a region with an electric
field of magnitude pointing in the positive y direction. The positive and negative ends of the dipole have charges
and , respectively, and the two charges are a distance apart. The dipole has moment of inertia about its
center of mass. The dipole is released from angle , and it is allowed to rotate freely.
Part A
What is , the magnitude of the dipole's angular velocity when it is pointing along the y axis?
Hint A.1 How to approach the problem
Hint not displayed
Hint A.2 Find the potential energy
Hint not displayed
Hint A.3 Find the total energy at the moment of release
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Hint not displayed
Hint A.4 Find the total energy when
Hint not displayed
Express your answer in terms of quantities given in the problem introduction.
ANSWER:
=
Correct
Thus increases with increasing , as you would expect. An easier way to see this is to use the trigonometric
identity
to write as .
Part B
If is small, the dipole will exhibit simple harmonic motion after it is released. What is the period of the
dipole's oscillations in this case?
Hint B.1 How to approach the problem
Hint not displayed
Hint B.2 Compute the torque
Hint not displayed
Hint B.3 The small-angle approximation
Hint not displayed
Hint B.4 Find the oscillation frequency
Hint not displayed
Hint B.5 The relationship between (angular) oscillation frequency and period
Hint not displayed
Express your answer in terms of and quantities given in the problem introduction.
ANSWER:
= Correct